The external surfaces of the telescope system were designed to minimize the main shell temperature and, therefore, heat loads to the cryogen. The sunshade protected the telescope aperture from solar radiation when the telescope was pointed more than 60° from the telescope-Sun line. The specular inner surface of the sunshade minimized heat loads into the dewar by presenting a cold surface (95 K) to the aperture and by reflecting away radiation from the Earth. The dewar's location behind the solar panel assembly also reduced the solar heat load. The Earth shield, located on the lower side of the telescope and facing away from the Sun, partially blocked terrestrial radiation, while the dewar wall opposite the solar panels radiated unwanted heat to deep space. A large multilayer insulation blanket between the spacecraft and the lower dewar-shell minimized heat flow in that area. The signal processing electronics boxes were mounted on low conduction composite trusses and surrounded by blankets to reduce heat input into the dewar. The cables connecting the focal plane outputs on the main shell to the exterior electronics boxes were fabricated of low thermal conductivity stainless steel coaxial cables. For further discussion of the thermal performance, see Urbach (1984).